ME 300 Engineering Thermodynamics Lecture 6 Second law I Second law of thermodynamics and some of its consequences Questions Understand the Kelvin Planck and the Clausius statement of the second law Understand the cyclic operation of heat engines heat pumps and refrigerators air conditioners How are thermal efficiency and coefficient of performance defined 1 INTRODUCTION TO THE SECOND LAW Recap and Intro 2 1st law of thermodynamics Conservation of energy principle Energy can neither be created nor destroyed but only converted from one form to another 2nd law of thermodynamics Increase of entropy principle The entropy of an isolated system during a process always increases or in the limiting case of a reversible process remains constant It never decreases Determines the direction of spontaneous change Can be used to determine the equilibrium state of any system e g chemical reactions ice cube melting iron bar rusting away etc Establishes theoretical limits of performance of energy conversion devices Both laws are based on experiments and observations Why we need the 2nd law What happens with hot coffee left in a room 3 1 Eventually cools off 2 Gets even hotter in the cool room as a result of heat transfer from the room air Either case does not violate the 1st law puts no limits on the inter conversion of energy Q or Q The Second Law of Thermodynamics Identifies the direction of a process 4 Entropy helps determining whether the reverse process violates the 2nd law Predicts the degree of completion of a chemical reaction The Second Law of Thermodynamics 5 th max 73 VS th max 17 The Second Law of Thermodynamics 6 Provides theoretical limits for the performance of engineering systems heat engines refrigerators etc Energy has a quality 2nd law as well as quantity 1st law Not all forms of energy are equally valuable Work can be directly and completely converted to other forms of energy i e heat But converting heat to work is not easy Requires the use of special devices such as heat engines 7 THE KELVIN PLANCK STATEMENT The Second Law of Thermodynamics Kelvin Planck statement 8 It is impossible to construct a cyclically operating device for which the only effect is exchange of heat with a single reservoir and the creation of an equivalent amount of work High temperature reservoir Work Heat reservoir 9 A heat reservoir is a source sink of heat so large that the release extraction of any desired amount of heat does not change the temperature of the reservoir For example an ocean the earth s atmosphere the Sun and two phase systems can be considered heat reservoirs Tearth 298K Tearth 298K Earth serves as heat reservoir Tice tea 273K Tcold tea 295K Heat engines 10 Energy is transported as heat from a high T reservoir to the heat engine which undergoes cyclic operation A portion is converted to work the remainder is lost in a low T reservoir High temperature reservoir TH Cyclic device Wnet out Work QH QL Low temperature reservoir TC Cyclic device device that operates in a thermodynamic cycle continual operation Working fluid returns to initial state Internal combustion engines Mechanical cycle 11 Examples Gas turbine Car engines External combustion engines Thermodynamic cycle Steam power plant 1st law analysis Lecture note p 2 12 High temperature reservoir TH Cyclic device Wnet out Work QH QL Low temperature reservoir TC Thermal efficiency Lecture note p 2 13 High temperature reservoir QH QL Wnet out Work Low temperature reservoir Ordinary SI engine 25 Diesel engine 40 Large gas turbine plants 40 Large combined gas steam power plants 60 2nd law aspects 14 High temperature reservoir QH QL Wnet Work Low temperature reservoir Never zero The cycle continuous operation cannot be completed without heat rejection 2nd law aspects 15 High temperature reservoir QH Wnet Work Repeated Kelvin Planck statement 16 It is impossible to construct a cyclically operating device for which the only effect is exchange of heat with a single reservoir and the creation of an equivalent amount of work High temperature reservoir th 100 Work Why do we need the 2nd law I 17 Establishes theoretical limits of performance of energy conversion devices th net W Q H Q H Q H Q L 1 Q L Q H Carnot efficiency later Ordinary SI engine 25 Diesel engine 40 Large gas turbine plants 40 Large combined gas steam power plants 60 Kelvin Planck statement Second Law Consequences Not all forms of energy are equally valuable 18 Energy has not only quantity 1st law but also quality 2nd law related aspects What we prefer work organized energy What is usually easy to get heat random energy Work can be converted completely to heat But heat cannot be completely and continuously converted into work This also requires the use of special devices such as cyclic heat engines The 1st law puts no limits on the interconversion Efficiency ability to convert random energy to organized energy Efficiency useful work provided heat 19 THE CLAUSIUS STATEMENT The Second Law of Thermodynamics Clausius statement 20 It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a low temperature body to a higher temperature body Book Heat generally cannot spontaneously flow from a material at lower temperature to a material at higher temperature High temperature reservoir Spontaneous No work applied The 1st law provides no guideline for the direction of spontaneous processes only stipulating that energy is conserved Low temperature reservoir Reversed heat engine 21 Room High temperature reservoir Reversed heat engine heat pump or refrigerator Wnet in Work QH QL Low temperature reservoir Atmosphere ground etc Refrigerated space Refrigerators 22 Heat transfer in general occurs from a high temperature medium to a low temperature medium The reverse process does not spontaneously occur In order to transfer heat from a low temperature medium to a high temperature medium a special cyclic device is required Refrigerator or air conditioner reversed cycle heat engine RT Heat pump 23 A heat pump operates on the same refrigerator cycle but differs in its objective Refrigerator To maintain the refrigerated space at low temperature by removing heat from it Heat pump To maintain heated space at a high temperature How does a heat pump in a motel work Heat pump operation Reversing valve Cooling mode 24 Q Q Heating mode Q Q 1st law analysis Lecture note p 3 25 Room High temperature reservoir Reversed heat engine heat pump